The present invention relates to plasma arc cutting torches and more particularly to a method and apparatus for initially positioning such a torch with respect to a workpiece.
Various methods and systems are known for exercising control over the position of a welding head during the welding process so as to maintain a proper arc length. Such methods are conventionally employed for consumable electrode welding and TIG (tungsten-inert gas) welding and typically sense arc voltage or arc impedance as a parameter upon which the height of the torch is predicated. Such systems are also adaptable for use in plasma arc cutting where an arc is generated through an ionizable gas jet to generate a high velocity plasma which actually cuts the workpiece.
The present invention is directed toward the problem of initially positioning a plasma arc torch before the arc is struck, i.e. the bringing of the torch head close enough to the workpiece so that the arc may be reliably struck without advancing the torch until it actually contacts the workpiece. The nozzle which constitutes the lowermost portion of the torch is typically constructed of a ceramic material which is both non-conductive and which can withstand the heat generated but which is relatively brittle and can be easily damaged. Typically, initial positioning of such torches has been accomplished under manual control of the operator who visually observes the height of the torch head relative to the workpiece. Such visual observation is difficult in situations where large plates are being cut and the torch may be some distance from the operator. Accordingly, initial positioning under manual control based on visual observation has frequently resulted in damage to the ceramic torch nozzle when the torch inadvertently contacts the workpiece.